Generally, a power steering system is either hydraulic or motor-driven. In hydraulic power steering systems that employ oil, a power device (e.g., power pump) pumps hydraulic fluid by using engine power and generates hydraulic pressure. The power device then applies the pressure in the direction in which a driver turns the steering wheel, thus aiding the driver's effort in turning the wheels.
The Motor-Driven Power Steering (MDPS) system multiplies the driver's steering force by using an electric motor. The force required to steer a vehicle is inversely proportional to the speed of the vehicle. Therefore, when the vehicle moves at a low speed, the MDPS is designed to deliver a significant amount of current to the motor to increase the steering assist force. When the vehicle moves at a moderate speed, the MDPS delivers a moderate, predetermined amount of current to the motor. However, when the vehicle moves at a high speed, the current is isolated to eliminate current flowing to the motor. Thus, the motor provides no steering assist force to the driver at high speeds.
However, there is a drawback in the conventional motor-driven power steering systems in that a motor to obtain sufficient rack driving force in mid-sized or heavy vehicles is not easily developed or controlled. Furthermore, the steering is executed by the driving force of the motor with a rack and pinion, thereby increasing energy loss.